1. Technical Field
The present invention is in the field of phase locked-loop (PLL)-based frequency synthesizers. More specifically, the present invention provides a PLL-based frequency synthesizer with a sub-sampling feedback loop that is particularly well-suited for use in a wireless device.
2. Description of the Related Art
The frequency synthesizer is an important element in any wireless device. It is responsible for generating a sinusoidal output signal with an accurate frequency that is used to translate the frequency band of the information channels transmitted by the wireless device. By tuning the frequency synthesizer, channel selection among the information channels is achieved. The spectral purity of the frequency synthesizer""s output signal has an effect on the information signal selected, which is typically converted down to baseband or to an intermediate frequency. The spectral purity of this output signal is characterized by the amount of spurious signals and the amount of phase-noise.
FIG. 1A sets forth a prior art PLL-based frequency synthesizer utilizing a voltage-controlled oscillator (VCO) 12 configured in a feedback loop. In this circuit, the frequency output of the VCO (f0), which is the output signal from the frequency synthesizer, is divided down to a low frequency using a 1/N divider circuit 14, and then fed back to the VCO 12 input through circuits 16, 18. The divider 14 output signal is coupled to one input of a phase-frequency detector (PFD) 16. A fixed reference frequency, fref, is coupled to the other input of the PFD 16. The output of the PFD 16 is coupled to and filtered in a loop filter 18, which sets the bandwidth and spurious rejection of the circuit. The output of the filter 18 is then coupled to the input of the VCO 12.
In the PLL shown in FIG. 1A, the reference frequency (fref) is fixed and is derived from an external crystal oscillator (not shown). Tuning in this prior art circuit is achieved by changing the division ratio N. Since N is an integer, the tuning step will be equal to fref. Hence, fref must be chosen to be equal to the desired channel spacing according to whatever wireless standard is associated with the wireless device. Due to spurious rejection considerations, the loop bandwidth (BW) is typically limited to approximately fref/10. Having such a small bandwidth, this prior art circuit generally does not inhibit the VCO""s phase-noise over the frequency range of interest. It is only by increasing the power of the VCO 12, and/or using an off-chip VCO, that the desired phase-noise performance can be attained in this prior art scheme.
A major source of phase-noise in such a PLL 10 is the VCO 12. The phase-noise of a free-running VCO is generally inversely proportional to its power consumption and drops quadratically with the offset from the circuit""s center frequency. When configured in a PLL, the phase-noise of the VCO is inhibited within the bandwidth (BW) of the PLL. This is due to the fact that, within it""s BW, the PLL corrects for any jitter occurring in the VCO. Thus, in order to reduce the phase-noise caused by the VCO, it is desired to increase its BW. However, this is in conflict with the other objective of maintaining adequate spurious rejection.
The spurious tone in a PLL arises from the input reference frequency. This is due to the fact that the PFD 16 in FIG. 1A produces pulses every cycle of the input clock. Unless the loop bandwidth is small enough to reject it, this frequency will appear at the output of the PLL as discrete tones, at an offset from the carrier equal to fref. Hence, the bandwidth of the PLL is usually a decade less than the reference frequency.
The known architecture in FIG. 1A also has problems because of the division ratio, N. This is so because when the phase-noise in the reference signal is mapped to the output, it is multiplied by a factor of N2 (or, equivalently, increased by 20 log(N) dB). Since N is usually very large (e.g., 5,000-70,000) in such PLLs, even a relatively xe2x80x9ccleanxe2x80x9d input signal may contaminate the output spectrum.
A traditional alternative to the PLL of FIG. 1A is a fractional-N architecture, which relies on periodically hopping the division ratio (N) between two consecutive integers such that the average resulting division ratio is a fraction. While this results in a moderate reduction of the division ratio (e.g., factor of 8 or 16), a spur at the channel spacing still persists, necessitating a low BW.
FIG. 1B sets forth another prior art circuit architecture 10xe2x80x2 for a phase-locked loop using a VCO. Similar to the first prior art circuit of FIG. 1A, in FIG. 1B a VCO 12xe2x80x2 is configured in a feedback loop. The output of the VCO (f0) is first divided down to an intermediate frequency using a 1/N divider circuit 14xe2x80x2, where N is an integer. The divider 14xe2x80x2 outputs a signal that is input to a PFD 16xe2x80x2. A reference frequency, frefxe2x80x2 is also input to the PFD 16xe2x80x2. The output from the PFD 16xe2x80x2 is fed to and filtered in a loop filter 18xe2x80x2, which sets the bandwidth and spurious rejection of the circuit. The filter 18xe2x80x2 then outputs a signal to the VCO 12xe2x80x2. In this second prior art circuit, however, the reference frequency (frefxe2x80x2) is not fixed as in FIG. 1A, but is a tunable frequency that is derived from a digital frequency synthesizer (DDS) 20xe2x80x2, or some other type of frequency synthesizer. Using this architecture, the reference frequency (frefxe2x80x2) can be made much larger than the channel spacing and the BW proportionally increased.
This architecture, however, suffers from several problems. The primary problem relates to the selection of the division ratio N. If the division ratio N is small, then the DDS 20xe2x80x2 will have to operate at a relatively high frequency, which leads to excessive power consumption in the circuit. This high frequency operation also leads to greater difficulty in designing the digital-to-analog convertor (DAC) at the DDS output. If the division ratio N is large, then the phase-noise of the DDS 20xe2x80x2 will start to dominate since it is multiplied by N2 when mapped to the output. Thus, the problem of phase-noise found in the prior art system in FIG. 1A is transferred in this second prior art system shown in FIG. 1B from the VCO 12 to the DDS 20.
One solution to this problem is to mix the VCO signal with the output of another PLL, thus converting down the VCO signal to a low frequency while keeping the division ratio small. However, this requires the use of a mixer operating at a high RF frequency and the use of two PLLs, which leads to high power consumption. Also, the output of the second PLL must be xe2x80x9ccleanxe2x80x9d since it will affect the phase-noise in the frequency synthesizer output.
A PLL-based frequency synthesizer is provided. In accordance with one aspect, a phase locked loop is provided that comprises a phase detector, a loop filter, a voltage controlled oscillator, and a feedback circuit. The phase detector has as inputs a reference frequency signal and a feedback signal. The phase detector is operable to generate a phase detection signal based on a comparison of phases between the reference frequency signal and the feedback signal. The loop filter is coupled to the phase detector for receiving the phase detection signal and generates an output voltage in response to the phase detection signal. The voltage controlled oscillator is coupled to the output voltage of the loop filter and generates a local oscillator signal. The feedback circuit is coupled to the local oscillator signal and generates the feedback signal. The feedback circuit comprises a sampling circuit. The sampling circuit is operable to sample the local oscillator signal with a sampling frequency that is substantially less than the frequency of the local oscillator signal to generate a sampling circuit output signal. The sampling circuit output signal comprises a beat frequency signal having a beat frequency that is equal to the frequency difference between the frequency of the local oscillator signal and the nearest harmonic of the sampling frequency. The feedback signal is derived from the beat frequency signal.